In recent years, a radio terminal such as a mobile phone and a wireless LAN (Local Area Network) is remarkably spreading and the frequency band used is also extending up to GHz band. Therefore, it is now difficult to find out new frequency bands to be used. In the background explained above, attention is now focused on the communication systems using impulse trains formed of pulses having extremely narrow pulse width (f or example, about 1 ns) which have been proposed as the novel methods to use the frequency resources. Such communication system using the pulse trains include, for example, an Ultra-Wideband Impulse Radio (hereinafter, referred to as “UWB-IR”) communication system. As an example of this communication system, the UWB-IR communication system wherein the Gaussian mono-pulse is modulated with the PPM (Pulse Position Modulation) system is disclosed in “Impulse Radio: How It Works” by Moe Z. Win, IEEE Communications Letters, Vol. 2, No. 2, pages 36 to 38 (February 1998).
Moreover, as a modulation system in the UWB-IR communication system, the direct sequence spread spectrum for spreading pulse trains with the spread code is employed. In this case, a plurality of spread pulses correspond to one data value. An example of the direct sequence spread spectrum type UWB-IR communication apparatus is disclosed in Japanese Patent Laid-Open Nos. 2002-335189 and 2002-335228.